Chapter 5 -- Measurement of Small Signals -- Overview
5-1
Chapter 5
-- Measurement of Small Signals
Overview
The PCI4 is a sensitive scientific instrument. It can resolve current changes as small as 0.01 picoamps (10
-14
amps). To place this current in perspective, 0.01 pA represents the flow of about 60,000 electrons per second!
The small currents measured by the PCI4 place demands on the instrument, the cell, the cables and the
experimenter. Many of the techniques used in higher current electrochemistry must be modified when used to
measure pA currents. In many cases, the basic physics of the measurement must be considered.
This chapter will discuss the limiting factors controlling low current measurements. It will include hints on cell
and system design. The emphasis will be on EIS (Electrochemical Impedance Spectroscopy), a highly
demanding application for the PCI4.
Measurement System Model and Physical Limitations
To get a feel for the physical limits implied by picoamp measurements, consider the equivalent circuit shown in
Figure 5-1. We are attempting to measure a cell impedance given by Z
cell
.
This model is valid for analysis purposes even though the real PCI4 circuit topology differs significantly.
In Figure 5-1:
E
s
Is an ideal signal source
Z
cell
Is the unknown cell impedance
R
m
Is the current measurement circuit's current measurement resistance
R
shunt
Is an unwanted resistance across the cell
C
shunt
Is an unwanted capacitance across the cell
C
in
Is the current measurement circuit's stray input capacitance
R
in
Is the current measurement circuit's stray input resistance
I
in
Is the measurement circuit's input current
In the ideal current measurement circuit R
in
is infinite while C
in
and I
in
are zero. All the cell current, I
cell
, flows
through R
m
.
With an ideal cell and voltage source, R
shunt
is infinite and C
shunt
is zero. All the current flowing into the
current measurement circuit is due to Z
cell
.
The voltage developed across R
m
is measured by the meter as V
m
. Given the idealities discussed above, one
can use Kirchoff's and Ohms law to calculate Z
cell
:
Z
cell
= E
s
* R
m
/ V
m
Содержание PCI4/300 Potentiostat/Galvanostat/ZRA
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